Support Ruby 4.0

The logger gem used to be a default gem, but is now a bundled gem which
means it needs to be declared as an explicit dependency.

Author: ekohl

.github/matrix.json

@@ -1,3 +1,3 @@
 {
-  "ruby": ["3.0", "3.1", "3.2", "3.3", "3.4"]
+  "ruby": ["3.0", "3.1", "3.2", "3.3", "3.4", "4.0"]
 }

smart_proxy.gemspec

@@ -15,6 +15,7 @@ Gem::Specification.new do |s|
   s.required_ruby_version = '>= 3.0'
   s.add_dependency 'base64'
   s.add_dependency 'json'
+  s.add_dependency 'logger'
   s.add_dependency 'logging'
   s.add_dependency 'ostruct'
   s.add_dependency 'rack', '>= 1.3'